Distributed energy resource (DER) units connected to distribution networks have potential to reduce demand for distribution and transmission network capacity, reduce losses and also increase the reliability of electricity supply to customers. Also, interconnection IEDs of distributed generation (DG) should have enabling functionalities to support active management of future distribution networks and support the realization of vision from environmentally friendly, energy efficient and reliable electricity distribution systems; i.e., Smart Grids. One of the functionalities required from a DG interconnection IED is a reliable detection of islanding, also called loss-of-mains (LOM) or anti-islanding protection.
Techniques proposed for islanding detection can be generally divided into two categories: communication based methods and local detection based methods. The local detection methods include so called active methods, where the measurement points send some sort of test signal to the network. An example of such a method is disclosed in EP 1764 894. Local detection methods also include passive methods, where the measurement point looks for transient events in the network.
The benefits of communication based LOM protection are lack of non-detection zone (NDZ) near power balance situation and lack of unwanted DG trips; that is, nuisance tripping due to other network events. However, the communication based LOM detection requires high-speed operation. The main challenges of communication based transfer trip LOM schemes are availability and cost of high-speed communication as well as flexibility to network topology changes. However, if communication based methods are not available, or communication fails or is not high-speed enough, a reliable LOM method based on local detection is still required in the future, but it has to overcome the major drawbacks of traditional LOM methods.
Active local methods have in some cases been questionable, because they introduce disturbances into the distribution network which may become a serious problem when the number of DG units increases in future.
The main focus in recent years has been on the communication based methods and the active methods, and the study of passive methods has been reducing due to a belief that they could not fulfill the demands in terms of speed, reliability, and avoiding unnecessary trip decisions.
Passive methods are based on monitoring one or more system parameters locally and making a trip decision without directly interacting with the network. Known passive LOM detection methods include monitoring of frequency (f), rate-of-change-of-frequency (ROCOF/df/dt), vector shift (VS), phase jump or voltage (U). The main issues with these known methods are failure to detect islanding near power balance and nuisance tripping of DGs.
ROCOF is a commonly employed passive LOM detection technique. However, the security of ROCOF relays is continually questioned, as it is sensitive to network disturbances, leading to nuisance tripping.
This method is highlighted by way of a simulation result of FIG. 5. The x-axis shows the time in seconds, and the y-axis shows the rate of change of a frequency parameter. The axis further shows some network events. At the time moment 0.6-0.9 s, an islanding condition occurs and at 2.3 s, capacitor switching occurs at a HV/MV substation. Three faults are also shown. At 3.0-3.15 s, there is a 1-phase-to-earth fault of another feeder, at 3.9-4.05 s there is a 1-phase-to-earth fault of the same feeder, and at 4.8-4.95 s there is a 3-phase fault in another feeder.
If it is assumed that the threshold setting would be in 6 Hz/s, ROCOF would cause a tripping in the other four events, but not in the desired one that is in the islanding event.
In ROCOF-based protection, compromise often has to be made between the relay's security and dependability. Reducing the trip threshold can reduce the non-detection zone; that is, more events in the network are detected. This approach, however, could make the relays too sensitive, resulting in more nuisance trips. As FIG. 5 shows, however, finding a proper threshold value is practically impossible.
As it is difficult to select a single criterion that would work in most situations, some multi-criteria approaches have been proposed. One multi-criteria approach has been disclosed in US 2010/0286838, which uses ROCOF and frequency as parameters. The proposed method basically, however, suffers from the same issues as the single ROCOF; that is, it can be difficult to find correct settings for the threshold values such that the operation would be sensitive enough but would avoid nuisance tripping.
Thus, a reliable, local measurement based island detection algorithm without NDZ and nuisance tripping of DG units would be desirable, along with improvements to the current passive methods.